This invention is related to the analysis of milk. Specifically, this invention is related to a device and method of determining the protein and fat content of milk for the purpose of producing milk with a predetermined ratio of protein and fat. More specifically, the present invention is related to a device and method for continuously monitoring the fat and micellar associated protein (MAP) content of milk, separated milk fractions, and reconstituted milk products in commercial type milk processing systems, and for controlling the ratio of fat to MAP in an output stream of these systems.
Milk consists primarily of water containing proteins, carbohydrates, electrolytes, lipids, and vitamins. The major proteins of milk are categorized as caseins, lactalbumins, and lactoglobulins, which include immunoglobulins. Milk lipids are mainly triacylglycerides containing the residues of various saturated and unsaturated, branched and straight chain fatty acids. In addition, some free fatty acids and phospholipids are also present.
In freshly secreted milk, lipids occur as butterfat globules. The fat globules, initially ranging in size from 1 to 15 um mean diameter, quickly begin to coalesce and separate under the force of gravity. Once the butterfat has separated out, the remaining fraction is skim milk.
Of the proteins, the lactalbumins and lactoglobulins (commonly known as whey proteins) are mostly soluble, while the caseins and a minor fraction of the other proteins take the form of complex micelles. Micelles and micelle-associated proteins (MAP) give skim milk its white appearance. The addition of detergent chelaters, such as ethylenediaminetetraacetic acid (EDTA), break down the micelles and render skim milk essentially clear.
The making of cheese is based on the coagulation of casein and the other MAPs from milk. The casein is precipitated from the whey by acidification, which can be accomplished by natural culturing, or the addition of rennet. The procedures for making cheese vary greatly resulting in numerous cheese products. The composition and handling of the original milk, bacterial flora, and starter culture provide the basic variables in cheese making. These basic variables, along with heat treatments, flavoring, salting, forming, and curing, determine the characteristics of the final product.
The type of cheese as well as the efficiency of production is critically influenced by the ratio of MAP to butterfat in the milk. Consequently, commercial dairy milk for successful cheese production must meet the specifications for fat-to-MAP ratio of the cheese manufacturer purchasing the milk. These specifications are often difficult to meet since each variety of cheese calls for a different fat-to-MAP ratio.
Numerous laboratory methods for determining fat and protein content in milk are known in the art. Fat concentration is routinely determined using the Babcock test or one of its modifications. Alternatively, other assay methods are employed, including the Mojonnier method, the Gerber test, and the TeSa test. In: "Laboratory Manual, Methods of Analysis of Milk and its Products", The Milk Industry Foundation, Washington D.C., 1959.
The protein content of milk and milk products can be determined by a variety of laboratory methods including gasometric assays, variations of the Kjedahl method, quantitative titration analysis, colorimetric assay, and dye binding and spectrophotometric optical analysis. However, these methods are too complex for routine use in a dairy plant. Consequently, measurement of milk protein content has seldom been used as a basis for commercialization in spite of the fact that cheese production is based on casein content.
Additionally, laboratory methods, adequate for commercial operations based on batch processing, are far too slow to be used with the current more commonly used continuous-flow systems. Continuous-flow equipment, which allows high product throughput, requires continuous rapid measurement of fat and protein content coupled with rapid feedback controls based on the monitored results. In these instances laboratory sampling is unreliable because the composition of the milk is constantly changing and the test sample becomes irrelevant by the time the laboratory results are complete.
A simple monitoring/control device responsive to fat content in continuous flow systems is known in the art. U.S. Pat. No. 4,144,804 describes an "Apparatus for Continuously Controlling Butterfat Content of Reconstituted Milk". U.S. Pat. No. 4,144,804 specifies an apparatus wherein milk is continuously sampled and diluted with a detergent chelator. The detergent chelator renders all components of the diluted milk transparent, except for the fat. The fat content is analyzed by absorbance of visible light in a spectrophotometric flow cell. In this system the butterfat content assay results are immediately available electronically and used as feedback for regulating valves that control the rate of butterfat reconstitution.
However, a simple system that is adaptable for monitoring and controlling protein content, or more specifically casein or MAP content, or casein or MAP to fat ratio, in continuous flow milk processing plants remains unknown in the art.